1. Field of the Invention
The present invention relates to a communication system for performing transmission and reception of data among a plurality of equipment and more particularly to a communication control apparatus for governing the transmission and reception of data.
2. Description of the Prior Art
A terminal handling system is known in the art in which a plurality of terminal equipment are connected to a control equipment, such as a terminal control equipment. Each of the terminal equipment includes output devices, such as a CRT display and a printer, and input devices, such as a keyboard. On the other hand, the terminal control equipment performs such operations as the processing of the data received from these input devices and a central processor forming a higher rank equipment and the editing of the data for delivery to the output devices. In this case, the transmission of data frequently occurs between the terminal control equipment and the terminal equipment. Generally, the half duplex bidirectional communication system is known as one method for the transmission of data between the two equipment.
In other words, each of the terminal equipment and the terminal control equipment includes a communication controller and the transmission of data is effected between the communication controllers.
FIG. 1 shows the communication controller of the terminal control equipment. In the terminal control equipment, a signal 101 generated from a transmission/reception control unit 11 is modulated by a modulating circuit 12 and then applied to pulse transformers 25 and 26 through driver circuits 14, 15 and 20, 21, respectively. The transformers 25 and 26 respectively generate RZI (return-to-zero-inverted) bipolar signals 104 and 105 which in turn are respectively transmitted to the terminal equipment through connectors 17 and 24 and coaxial cables. On the other hand, bipolar signals 104 and 105 received from the terminal equipment through the connectors 17 and 24 are respectively transmitted to the pulse transformers 25 and 26. These signals are then amplified by amplifier circuits 18 and 22, respectively. The resulting outputs 106 and 109 are applied to an OR circuit 19 and the resulting logical sum output 107 is applied to a demodulating circuit 13. The demodulating circuit 13 converts the RZI signal to a regular logic signal 108 and applies it to the transmission/reception control unit 11. Designated at V.sub.D is the drive voltage for the pulse transformers 25 and 26 and terminating resistors 16 and 23 have the same resistance value as the characteristic impedance of the coaxial cables so as to serve the function of suppressing the reflection of the transmitted and received signals.
FIG. 2 shows the communication controller in the terminal equipment.
In the terminal equipment, a signal 202 generated from a transmission/reception control unit 80 is modulated by a modulating circuit 81 and applied to a pulse transformer 85 through driver circuits 83 and 84. Thus, the pulse transformer 85 generates an RZI bipolar signal 205 and the signal 205 is transmitted to the terminal control equipment from a connector 87 through a coaxial cable. On the other hand, an RZI bipolar signal 205 received from the terminal control equipment through the connector 87 is transmitted to the pulse transformer 85. Then, the signal 205 is amplified by an amplifier circuit 86 and the resulting amplified output 206 is applied to a demodulating circuit 82. The demodulating circuit 82 converts the RZI signal to the regular logic signal 201 and applies it to the transmission/reception control unit 80. Designated at V.sub.D is the drive voltage for the pulse transformer 85. A connector 88 is provided so that the signal received at the connector 87 from the terminal control equipment through the coaxial cable is sent to any other terminal equipment within the terminal system. A terminating resistor 90 has the same resistance value as the characteristic impedance of the coaxial cables so as to serve the function of suppressing the reflection of transmitted and received signals. A terminal on/off switch 89 is provided so that it is turned on to make the terminating resistor 90 effective only when the corresponding terminal equipment is located at the extreme end within the terminal system.
FIG. 3 is a diagram for explaining a prior art technique for realizing the required half duplex bidirectional transmission between the terminal control equipment and the terminal equipment in the form of optical data communication. A terminal control equipment 31 includes two communication controllers of the type shown in FIG. 1 and it also includes four connectors 36 to 39. On the other hand, a plurality of terminal equipment 34 and 35 are each provided with a single communication controller of the type shown in FIG. 2 and also they are respectively provided with connectors 42, 43 and 44, 45. Optical data sets 32 and 33 are arranged between the terminal control equipment 31 and the terminal equipment 34, and of the connectors connected to the terminal control equipment 31 the connectors 36 and 37 are respectively provided for transmitting and receiving purposes. The connector 36 and 37 and connectors 40 and 41 provided on the side of the optical data set 32 are connected by coaxial cables 69 and 70, respectively. In the optical data set 32, the RZI bipolar signal applied through the connector 40 is received by a pulse transformer 50. Then, the signal is amplified by an amplifier circuit 53 whose output is applied to the input of an optical transmitter 61. The optical transmitter 61 serves the function of using for example a TTL-level signal to effect an LED on/off control and converting it into a light signal. This LED light output is sent to an optical receiver 64 of the optical data set 33 through an optical cord 73 whose ends are respectively connected to optical connectors 65 and 67. The optical receiver 64 serves the function of receiving the light signal by a photodiode and converting it to a TTL-level signal. The signal output of the TTL level is applied to the input of a data separation circuit 60. An RZI bipolar signal is generated by the data separation circuit 60, driver circuits 57 and 58 and a pulse transformer 52 and this signal is transmitted to the terminal equipment 34 through a connector 46, a coaxial cable 71 and the connector 42. Simultaneously, this bipolar signal is also transmitted to the terminal equipment 35 through the connector 43, a coaxial cable 72 and the connector 44. Also, the signal is similarly transmitted from the terminal equipment 35 to other terminal equipment through the connector 45 and a coaxial cable 75.
On the other hand, the RZI bipolar signal sent from the terminal equipment such as the terminal equipment 35 is applied to the optical data set 33 through the connector 44, the coaxial cable 72, the connector 42, the coaxial cable 71 and the connector 46. In the optical data set 33, this input signal is converted to a TTL-level signal by the pulse transformer 52 and an amplifier circuit 54 and the output from the amplifier circuit 54 is applied to an optical transmitter 62. This optical transmitter 62 functions in the same manner as the previously mentioned optical transmitter 61 so that the light output from the LED included in the optical transmitter 62 is transmitted to an optical receiver 63 in the optical data set 32 through an optical connector 68, an optical cord 74 and an optical connector 66. The optical receiver 63 serves the same function as the optical receiver 64 and its signal output of the TTL level is applied to the input of a data separation circuit 59. The data separation circuit 59 applies its output signal to a pulse transformer 51 through driver circuits 55 and 56. The pulse transformer 51 produces an RZI bipolar signal and the signal is transmitted to the terminal control equipment 31 through the connector 41, the coaxial cable 70 and the connector 37. In the Figure, designated at V.sub.D is the drive voltage for the pulse transformers 50 to 52. The connectors 38 and 39 of the terminal control equipment 31 are provided so as to be interfaced with another group of terminal equipment by coaxial cables. Terminating resistors 47 to 49 have the same resistance value as the characteristic impedance of the coaxial cables so as to serve the function of suppressing the reflection of transmitted and received signals. It is the usual practice to use the optical cards 73 and 74 in the form of two-core optical cable pairs.
The above-described prior art technique has the following disadvantages. The first disadvantage is that in the optical data set 33 the signal for driving the pulse transformer 52 is also applied to the amplifier circuit 54. Thus, there arises a situation in which the drive signal is looped back to the optical data set 32 and hence the drive signal collides with the transmitting data to the terminal control equipment 31 in the pulse transformer 51. This causes an excessive current flow in the pulse transformer 51 and the pulse transformer 51 is damaged. Thus, it is necessary to make modifications to the communication controller connected to the connector 37 of the terminal control equipment 31 so that the driver circuits 20 and 21 are eliminated from the construction of FIG. 1 and that the drive voltage V.sub.D applied to the middle point of the pulse transformer 26 is eliminated and the middle point is connected to ground. As a result, the existing standard terminal system cannot be changed into an optical communication system without modification.
The second disadvantage is that the construction of the optical data set differs depending on whether it is arranged on the side of the terminal control equipment or the terminal equipment and therefore it is necessary to develop two kinds of optical data set with the resulting increase in the development cost.
The third disadvantage is that the exclusive use of the two connectors of the terminal control equipment 31 reduces the number of terminal equipment to be connected in the system (by an amount for one trunk).